The present invention relates to a distributed information retrieval system, for example, to an image file retrieval system in which a central computer is connected to at least one image file retrieval device.
An image file retrieval device has functions for storing documents or the like as image data in an image file, for retrieving desired image data from the image file, and for displaying or printing the retrieved data. For example, in the prior art image file retrieval device as disclosed by the Japanese Patent Unexamined Publication No. 58-158769, control information containing retrieval keys to be subjected to retrieval processing with respect to an image data item is also stored in the image file retrieval device so that a set of related documents are obtained by achieving a retrieval on the basis of the control or management information.
For the function to extract a set of associated documents, common items such as a title and a keyword are included in each document and the common items are recorded as retrieval keys in the control information for the set of associated documents; therefore, such documents can be retrieved by use of the common items. In order to implement this function in an image file retrieval device, the control information necessary for the information retrieval must be configured in a flexible structure. That is, the addition and change to the control information must be easily accomplished, and a great number of control information items are required to be registered.
In a so-called distributed image file retrieval system in which the central computer is connected to a plurality of image file retrieval devices, local image data items are ordinarily stored on the image file retrieval device side, for example, to minimize the response time for forwarding an answer to the user of the retrieval system. However, there remains a problem to be solved, namely, on which one of the central computer and the terminal device should the management information be stored and should the retrieval processing be effected?
The control information related to the image data cannot be easily stored and accumulated in the image file retrieval device because of the following reasons. To enable distributed image data to be retrieved, all the control information must be stored in an image file retrieval device; however, the control information is of a considerably great amount. Even in a system in which only the control information related to local image data contained in an image file retrieval device is stored, namely, only a portion of the control information is stored, the data volume of the control information cannot be reduced to a satisfactory extent if the control information comprises detailed data items.
Moreover, data addition and change take place frequently on the control information as well as on the data format thereof. In the image file retrieval device, however, the flexible control and maintenance cannot be readily achieved in conjunction with such addition and change to the control information.
On the other hand, if the control information is stored on the central computer side to effect the entire retrieval processing by use of the central computer, the problem of the data volume and that of the flexible control and maintenance of the control information can be solved by the high-performance and large-capacity data base of the central computer. However, there still remains a problem for the user of the retrieval system. That is, in a retrieval operation, the user displays appropriate image data and confirms the displayed data to stepwise narrow the retrieval range including the objective data. Consequently, the retrieval processing is first achieved in a wide range covering a large volume of data, and thereafter it is executed in local narrow ranges many times. In a system in which the data retrieval is entirely performed by the central computer, a retrieval processing in a small range is frequently effected by the central computer, and hence an overhead time to be elapsed in the central computer in addition to the time used for the retrieval processing is increased, which prevents the effective use of the central computer. The response time is therefore increased in the retrieval. Moreover, this is also disadvantageous for the load distribution in the retrieval system.
In a distributed image file retrieval system as described above, if the control or management information is stored in the image file retrieval device, the flexibility of the system is deteriorated; and if it is stored in the central computer system, the inconvenience of the long response time is imposed on the user of the retrieval system. These problems also exist in general in a distributed information retrieval system in which the central computer is connected to retrieval terminal devices.